Heat Treatment System

ABSTRACT

A nitrogen enriched layer is formed in a primary heat treatment, and requenching is conducted in a secondary heat treatment, while the heat treatment efficiency is improved across the entire system. In a primary heat treatment device  1 , a bearing component is heated in a heater  11  at a temperature exceeding the A1 transformation point, and is then cooled in a cooler  12  to a temperature less than the A1 transformation point, thus forming a nitrogen enriched layer at the surface of the component. The bearing component that has undergone primary heat treatment is then subjected to high frequency heating in a heater  21  of a secondary heat treatment device  2 , at a temperature exceeding the A1 transformation point, and is then cooled in a cooler  22  to a temperature less than the A1 transformation point. Following cooling by the cooler  22 , the component is tempered using high frequency heating.

TECHNICAL FIELD

The present invention relates to a heat treatment system for conductinga two-stage heat treatment on a steel component.

BACKGROUND ART

An example of a heat treatment method that has been adapted for steelmachine components that require a long service life to rolling fatigue,such as the bearing portions of a rolling bearing, is disclosed inJapanese Patent Laid-Open Publication No. 2003-226918. This methodcomprises the steps of subjecting the steel for the bearing componentsto carbonitriding treatment at a carbonitriding treatment temperaturethat exceeds the A1 transformation point, subsequently cooling the steelto a temperature less than the A1 transformation point, and thenreheating and quenching the steel within a quenching temperature region(790 to 830° C.) exceeding the A1 transformation point but less than thecarbonitriding treatment temperature.

According to this method, the presence of a carbonitrided layer at thesurface of the steel increases the hardness of the bearing components,and because the quenching temperature during the reheating is restrictedto a temperature at which the growth of austenite crystal grains issuppressed, the average particle size of the austenite crystal grainscan be reduced to no more than 8 μm. As a result, the grain boundarystrength is increased, which produces effects such as an improvement inthe service life relative to rolling fatigue, and an improvement in thecracking resistance.

For example, the bearings used in reduction gears, drive pinions, andtransmissions and the like operate under severe conditions, includinghigh loads and contamination of the lubricant with foreign matter (suchas abrasion dust from the gears), and with recent demands for evenhigher speeds and greater miniaturization, the severity of theseoperating conditions continues to increase. In order to cope with theserecent trends, Japanese Patent Laid-Open Publication No. 2003-226918proposes a heat treatment method capable of producing bearing componentswith superior basic performance, including improved service liferelative to rolling fatigue, greater crack resistance and betterresistance to dimensional changes over time. This heat treatment methodcomprises the steps of subjecting the steel for the bearing componentsto carbonitriding treatment at a carbonitriding treatment temperaturethat exceeds the A1 transformation point, subsequently cooling the steelto a temperature less than the A1 transformation point, and thenreheating and quenching the steel within a quenching temperature region(790° C. to 830° C.) exceeding the A1 transformation point but less thanthe carbonitriding treatment temperature. According to this heattreatment method, the austenite crystal grains within the microstructureof the heat treated bearing components can be miniaturized to an averageparticle size of no more than 8 μm, enabling the production of bearingcomponents with superior basic performance, including improvements inthe service life relative to rolling fatigue, the Charpy impact value,the fracture toughness, and the compressive failure strength.

In the invention disclosed in the above publication, a total of two heattreatments, namely a primary treatment and a secondary treatment, areperformed, although tempering is necessary following the secondary heattreatment, in order to prevent quenching crack during the quenchingprocess. If the heating time for this tempering process is long, then itcannot be matched to the heating time of the secondary heat treatment,meaning the heat treated product must be halted within the productionline, which causes long down times for the machinery used to perform theprimary and secondary heat treatments, resulting in reduced heattreatment efficiency and longer heat treatment times. An object of thepresent invention is to improve the heat treatment efficiency across theentire system, while forming a nitrogen enriched layer during a primaryheat treatment, and then conducting requenching within a secondary heattreatment.

Furthermore, another object of the present invention is to provide aheat treatment system for conducting the above heat treatment method onsteel components such as bearing components, and in particular, toprovide a heat treatment system that is capable of subjecting each steelcomponent to a uniform heat treatment.

In the invention disclosed in the above publication, the heatingtemperature of the secondary heat treatment must be strictly controlledin order to ensure uniform miniaturization of the crystal grainsthroughout the whole component. If this secondary heating is conductedin an atmospheric furnace, in a similar manner to a conventionalquenching process, and the atmospheric temperature inside the furnace ismeasured, then differences can develop between the measured temperatureand the actual temperature of the bearing component, making stricttemperature control difficult. An object of the present invention is toprovide a heat treatment system for forming a nitrogen enriched layerduring a primary heat treatment, and then conducting requenching withina secondary heat treatment, wherein the heating temperature in thesecondary heat treatment is strictly controlled.

Furthermore, in the invention disclosed in the above publication, atotal of two heat treatments, namely a primary treatment and a secondarytreatment, are performed, but in those cases where the articleundergoing heat treatment is a particularly thin-walled member or amember of varying thickness, the occurrence of quenching distortionduring the heat treatment is a concern. Amongst the bearing componentswithin a rolling bearing, the outer ring and the inner ring arethin-walled members. In the case of a tapered roller bearing, since thethickness of the outer ring and the inner ring is also non-uniform,there is a real danger of quenching distortion. Moreover, in order toensure satisfactory bearing performance, since these bearing componentsrequire extremely high levels of dimensional precision, quenchingdistortion must be suppressed as far as possible. Another object of thepresent invention is to suppress quenching distortion of the steelcomponent, while forming a nitrogen enriched layer during a primary heattreatment, and then conducting requenching within a secondary heattreatment.

Furthermore, in the invention disclosed in the above publication, atotal of two heat treatments, namely a primary treatment and a secondarytreatment, are performed, but because the heating time required for theprimary heat treatment is different from that required for the secondaryheat treatment, the heating times for the primary heat treatment and thesecondary heat treatment cannot be easily balanced, and there is adanger that the heat treated product must be halted within theproduction line, causing longer down times for the machinery used toperform the heat treatments, and resulting in reduced heat treatmentefficiency. An object of the present invention is to improve the heattreatment efficiency across the entire system, while forming a nitrogenenriched layer during a primary heat treatment, and then conductingrequenching within a secondary heat treatment.

DISCLOSURE OF THE INVENTION

A heat treatment system according to the present invention comprises: aprimary heat treatment device for heating a steel component at atemperature exceeding the A1 transformation point and then cooling thecomponent to a temperature less than the A1 transformation point, thusforming a nitrogen enriched layer at the surface of the component; and asecondary heat treatment device for heating the steel component that hasundergone primary heat treatment, at a temperature exceeding the A1transformation point, and then cooling the component to a temperatureless than the A1 transformation point, wherein the secondary heattreatment device includes an induction heater, and tempering isperformed by induction heating following cooling within the secondaryheat treatment device.

According to this heat treatment system, the heat treatment by theprimary heat treatment device forms a nitrogen enriched layer in whichnitrogen is diffused throughout the component surface, thus increasingthe surface hardness of the steel component. On the other hand, theaustenite grains within the steel structure following the primary heattreatment are considerably large, but because the secondary heattreatment is subsequently conducted while controlling a heatingtemperature and a heating time by the subsequent induction heating, theaustenite grains can be reduced in size to approximately one half of thesize observed in conventional components, enabling a fine grain sizewith an austenite grain size number exceeding 10 to be achieved. As aresult of these characteristics, abrasion resistance and crackingresistance can be improved in comparison with conventional components,enabling a significant increase in the service life relative to rollingfatigue.

In the system of the present invention, both the heating of thesecondary heat treatment, and the tempering following the secondary heattreatment are conducted using induction heating. Compared with heatingin an atmospheric gas from a combustion furnace or the like, inductionheating offers the advantages of better heating efficiency and shorterheating times, and moreover because induction heating uses electricalenergy, control of the heating output is also very easy. Accordingly, byconducting both the heating of the secondary heat treatment, and thetempering following the secondary heat treatment using inductionheating, the heating times for the two heating steps can be balancedrelatively easily.

According to this aspect of the invention, a nitrogen enriched layer isformed in the primary heat treatment, and when requenching is thenconducted within the secondary heat treatment, the heating time for thesecondary heat treatment, and the heating time for the tempering stepfollowing the secondary heat treatment can be balanced easily.Accordingly, the need to halt heat treated products within theproduction line can be minimized, and the down times required for thevarious machinery can be reduced, enabling an improvement in the heattreatment efficiency across the entire system.

Furthermore, the present invention also provides a heat treatment systemcomprising: a primary treatment device for heating a steel component ata primary heating temperature exceeding the A1 transformation point andthen cooling the component to a temperature less than the A1transformation point, thus forming a nitrogen enriched layer at thesurface layer of the component; and a secondary treatment device forheating the steel component that has undergone heat treatment by theprimary treatment device, at a secondary heating temperature exceedingthe A1 transformation point by induction heating, and then cooling thecomponent to a temperature less than the A1 transformation point.

In the primary treatment device, the steel component is heated at aprimary heating temperature that exceeds the A1 transformation point,thus forming a nitrogen enriched layer in which nitrogen is diffusedthroughout the surface layer of the component, and the component is thencooled to a temperature less than the A1 transformation point.Subsequently, in the secondary treatment device, induction heating at asecondary heating temperature exceeding the A1 transformation point andquenching are conducted. Consequently, by controlling the heatingtemperature and the heating time, the austenite crystal grains withinthe micro structure of the heat treated steel component can be reducedin size, enabling a fine grain size to be achieved, with a grain sizenumber, determined in accordance with the austenite grain size testmethod prescribed in JIS G0551, exceeding 10. As a result, a steelcomponent with excellent service life relative to rolling fatigue, andsuperior levels of crack resistance and resistance to dimensionalchanges over time can be obtained.

Furthermore, the quenching of the steel component in the secondarytreatment device is performed piece by piece using an induction heatingsystem (such as high frequency quenching), and consequently bothnon-uniformity in the heat treatment quality within each individualsteel component, and variations in the quality of the heat treatmentacross different steel components can be minimized, enabling uniformsteel components of high reliability to be produced. In the secondarytreatment device, die quenching may be conducted following heating ofthe steel component to the secondary heating temperature using theinduction heating system. In this description, die quenching refers to atreatment method wherein quenching is conducted with the heated articleconstrained by a die, and includes press quenching in which the articleis constrained by applying pressure to the die.

In the primary treatment device, appropriate methods for dispersingnitrogen within the surface layer of the steel component to form anitrogen enriched layer include nitridation and carbonitridation,although considering the associated heating temperatures and the need toprevent decarburization, carbonitridation is preferred. Moreover, interms of cost and quality, gas carbonitriding is preferred.

The primary treatment device and the secondary treatment device eachhave a basic structure comprising a heater for heating a steel componentto a desired temperature (the primary heating temperature or thesecondary heating temperature), and a cooler for subsequently coolingthe component. For example, in those cases where gas carbonitriding isconducted in the primary treatment device, a heating furnace in whichthe steel component is heated within an atmospheric gas comprising acarburizing gas containing added ammonia is used as the heater for theprimary treatment device. This heating furnace may be either acontinuous-type or batch-type furnace. The heater of the secondarytreatment device is a heater that uses inductive heating (such as highfrequency heating) to heat the steel component, and is constructed froma high frequency heating device. There are no particular restrictions onthe cooling system used for the coolers of the primary treatment deviceand the secondary treatment device, and suitable cooling methods thatcan be adopted include air cooling, gas cooling using a gas such as N₂,oil cooling, water cooling, and salt bath cooling.

According to this aspect of the present invention, because the quenchingof the steel component in the secondary treatment device is performedpiece by piece using an induction heating system (such as high frequencyquenching), both non-uniformity in the heat treatment quality withineach individual steel component, and variations in the quality of theheat treatment across different steel components can be minimized. Thus,steel components with excellent service life relative to rollingfatigue, superior levels of crack resistance and resistance todimensional changes over time, and uniformly high reliability can beobtained.

Furthermore, a heat treatment system according to the present inventioncomprises: a primary heat treatment device for heating a steel componentat a temperature exceeding the A1 transformation point and then coolingthe component to a temperature less than the A1 transformation point,thus forming a nitrogen enriched layer at the surface of the component,and a secondary heat treatment device for heating the steel componentthat has undergone primary heat treatment, at a temperature exceedingthe A1 transformation point, and then cooling the component to atemperature less than the A1 transformation point, wherein inductionheating is conducted in the secondary heat treatment device, thetemperature of the steel component undergoing induction heating isdetected, and the induction heater is operated under feedback controlbased on the detected temperature value.

In this aspect of the present invention, induction heating such as highfrequency heating is conducted in the secondary heat treatment device,the temperature of the steel component undergoing induction heating isdetected, and the induction heater is operated under feedback controlbased on the detected temperature value, and consequently the secondaryheating temperature can be held reliably and precisely within a narrowtemperature range based on the actual temperature of the steelcomponent, enabling the production of a high quality steel component inwhich the crystal grain size has been reduced uniformly throughout theentire component.

In this case, in order to absolutely minimize any temperature errors,the temperature of the steel component undergoing induction heating ispreferably detected using a non-contact type temperature sensor.

As described above, according to this aspect of the present invention, anitrogen enriched layer is formed in the primary heat treatment, andwhen requenching is then conducted within the secondary heat treatment,the heating temperature within the secondary heat treatment device isable to be controlled with good precision. Accordingly, heatingirregularities within the secondary heat treatment can be prevented,enabling the crystal grain size to be reduced uniformly throughout theentire component, thus ensuring a more stable quality level for thesteel component.

Furthermore, a heat treatment system according to the present inventioncomprises: a primary heat treatment device for heating a steel componentat a temperature exceeding the A1 transformation point and then coolingthe component to a temperature less than the A1 transformation point,thus forming a nitrogen enriched layer at the surface of the component;and a secondary heat treatment device for heating the steel componentthat has undergone primary heat treatment, at a temperature exceedingthe A1 transformation point, and then cooling the component to atemperature less than the A1 transformation point, wherein the secondaryheat treatment device conducts induction heating and die quenching.

In this system of the present invention, as described above, bothinduction heating and die quenching are conducted within the secondaryheat treatment device, and consequently a heat treated product withlittle distortion and good dimensional precision can be obtained, andeven thin-walled components or components with varying thickness can beproduced with favorable dimensional precision.

According to this aspect of the present invention, a nitrogen enrichedlayer is formed in the primary heat treatment, and when requenching isthen conducted within the secondary heat treatment, a steel componentwith little thermal distortion and a high level of dimensional precisioncan be produced at low cost. In particular, the invention can also beideally applied to thin-walled components or components with varyingthickness.

Furthermore, a heat treatment system according to the present inventioncomprises: a primary heat treatment device for heating a steel componentat a temperature exceeding the A1 transformation point and then coolingthe component to a temperature less than the A1 transformation point,thus forming a nitrogen enriched layer at the surface of the component,and a secondary heat treatment device for heating the steel componentthat has undergone primary heat treatment, at a temperature exceedingthe A1 transformation point, and then cooling the component to atemperature less than the A1 transformation point, wherein a pluralityof secondary heat treatment devices are disposed in parallel.

In this system of the present invention, as described above, a pluralityof secondary heat treatment devices are disposed in parallel, andconsequently the secondary heat treatment can be conductedsimultaneously at a plurality of different locations, meaning the heattreatment efficiency of the secondary heat treatment can be improved.

In such cases, induction heating is preferably conducted within each ofthe secondary heat treatment devices. Compared with heating in anatmospheric furnace such as a combustion furnace, induction heatingoffers better work efficiency, and heating can be completed within ashorter time, meaning that by conducting such induction heating at aplurality of locations in parallel, the heating efficiency of thesecondary heat treatment can be improved dramatically. Accordingly, theheat treatment efficiency within the primary heat treatment and thesecondary heat treatment can be balanced, enabling an improvement in theheating efficiency across the entire system.

In this case, if die quenching is conducted within the secondary heattreatment devices, then heat treated products with little distortion anda high level of dimensional precision can be produced, and favorabledimensional precision can be ensured even for thin-walled components orcomponents of varying thickness.

According to this aspect of the present invention, a nitrogen enrichedlayer is formed in the primary heat treatment, and when requenching isthen conducted within the secondary heat treatment, the heatingefficiency within the secondary heat treatment can be improveddramatically. Accordingly, the heat treatment efficiency within theprimary heat treatment and the secondary heat treatment can be balanced,enabling an improvement in the heating efficiency across the entiresystem.

In each of the heat treatment systems described above, the method usedfor forming the nitrogen enriched layer in the primary heat treatment ispreferably carbonitridation, and in terms of cost and quality, gascarbonitriding is preferred. Gas carbonitriding can be conducted in anatmospheric furnace using an atmospheric gas comprising a carburizinggas containing added ammonia.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a schematic illustration of aheat treatment system according to the present invention;

FIG. 2 is a cross-sectional view showing a deep groove ball bearing;

FIG. 3 is a heat treatment cycle diagram;

FIG. 4 is a diagram showing a schematic illustration of the constructionof a heat treatment system according to the present invention;

FIG. 5 is a heat treatment cycle diagram for a heat treatment system;

FIG. 6 is a cross-sectional view showing a schematic illustration of aheat treatment system according to the present invention;

FIG. 7 is a cross-sectional view showing a schematic illustration of aheat treatment system according to the present invention;

FIG. 8 is a cross-sectional view of a tapered roller bearing; and

FIG. 9 is a cross-sectional view showing a schematic illustration of aheat treatment system according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

As follows is a description of a first embodiment of the presentinvention, applied to a bearing component that represents one example ofa steel component.

FIG. 1 shows a schematic illustration of the construction of a heattreatment system according to the present invention. As shown in thefigure, this heat treatment system comprises a primary heat treatmentdevice 1, a secondary heat treatment device 2, two washing devices 5 and6, and a tempering device 7. A bearing component produced by a moldingprocess (not shown in the drawings) comprising, for example, forging,and then turning, is fed sequentially through the primary heat treatmentdevice 1 and the secondary heat treatment device 2, and undergoesheating and cooling within each of the devices, as the primary heattreatment and the secondary heat treatment, respectively.

The term “bearing component” refers to a bearing component of a rollingbearing such as a ball bearing, a tapered roller bearing, a rollerbearing or a needle roller bearing. As one example, FIG. 2 shows a deepgroove ball bearing 4 comprising, as the main structural elements, anouter ring 41, an inner ring 42, and rolling elements (balls) 43, and ofthese structural elements, in this description, the term “bearingcomponents” describes those elements that are subjected to rollingcontact with an opposing member, namely the outer ring 41, the innerring 42 and the rolling elements 43. Examples of the materials that canbe used for these bearing components include bearing steel such as SUJ2prescribed in the JIS, as well as high temperature bearing steelcomprising C: 0.6 to 1.3 wt %, Si: 0.3 to 3.0 wt %, Mn: 0.2 to 1.5 wt %,Cr: 0.3 to 5.0 wt %, and Ni: 0.1 to 3 wt % (and preferably alsocomprising Mo: 0.05 to 0.25 wt % and V: 0.05 to 1.0 wt %), and mediumcarbon steel comprising C: 0.4 to 0.8 wt %, Si: 0.2 to 0.9 wt %, Mn: 0.7to 1.3 wt % and Cr: no more than 0.7 wt %.

The primary heat treatment device 1 comprises a heater 11 and a cooler12. In FIG. 1, a continuous system is shown for the heater 11, but abatch-type furnace can also be used. The heater 11 is formed, forexample, from an atmospheric furnace that uses an atmospheric gascomprising a carburizing gas containing added ammonia. Inside thisheater 11, a bearing component is heated at a temperature T1 (from 800°C. to 900° C., for example, 850° C.) exceeding the A1 transformationpoint for a predetermined time of 40 minutes for example (primaryheating), as shown in FIG. 3. This heating causes activated nitrogen todiffuse into the surface layer of the bearing component, thus hardeningthat surface layer (gas carbonitriding). The primary heating performedby the heater 11 has a primary object of forming a nitrogen enrichedlayer at the surface of the component, and at least nitridation mustoccur, although carburization is not essential. However, depending onthe conditions, carburization may be essential as well as nitridation,particularly in cases where decarburization is a concern, or cases wherethe carbon content of the steel is insufficient and a satisfactory levelof hardness cannot be achieved. The heater 11 may also use a vacuumfurnace, a salt bath furnace or an induction heater or the like.Following heat treatment, the bearing component is cooled to below theMs point by the cooler 12 (using oil cooling, for example), and is thentransported to a washing device 5 for washing and removal of the coolingliquid.

As shown in FIG. 1, a bearing component that has undergonecarbonitriding in the primary heat treatment device 1 is then suppliedto the secondary heat treatment device 2. The secondary heat treatmentdevice 2 comprises a heater 21 for performing induction heating such ashigh frequency heating, and a cooler 22. The bearing component suppliedto the heater 21 is positioned with a suitable clearance from aninductor (not shown in the figure), and as shown in FIG. 3, is thensubjected to induction heating (secondary heating), at a secondaryheating temperature T2 (for example, 880° C. to 900° C.) exceeding theA1 transformation point for a predetermined time (for example, 1.5 to 2seconds), by electrically conducting the inductor. FIG. 3 shows atemperature exceeding the A1 transformation point but less than theprimary heating temperature T1 in the primary heat treatment device 1 asthe secondary heating temperature T2; however, the upper limit of thesecondary heating temperature T2 may exceed T1. In the inductionheating, the heating temperature and the heating time can be preciselycontrolled and the treatment can be conducted within a short time.Consequently, the austenite crystal grains within the microstructure ofthe bearing components can be miniaturized. At this time, whether or notthe austenite crystal grains are miniaturized can be evaluated by aproduct of the heating temperature and the heating time. For example, ifthe maximum heating temperature in an induction heater is low, theaustenite crystal grains can be miniaturized by prolonging the heatingtime.

Following completion of this heating, the bearing component istransported to the cooler 22 and is cooled to below the Ms point (usingoil cooling, for example) to effect quenching. Instead of transportingthe bearing component to a cooler 22 that is separate from the heater 21as in the above example, the bearing component may also be subjected tospray cooling while still positioned at the induction heating locationinside the heater 21.

Following completion of the above secondary heat treatment, the bearingcomponent is washed in the washing device 6 to remove the coolingliquid, and is then transported to the tempering device 7 where, asshown in FIG. 3, tempering is conducted at a suitable temperature T3(180° C. for example). This tempering is conducted using inductionheating such as high frequency heating.

In the description above, oil cooling was used as the cooling methodwithin the primary heat treatment device 1 and the secondary heattreatment device 2, but other cooling methods such as water cooling, aircooling or gas cooling can also be employed, and different coolingmethods may also be used for the primary heat treatment device 1 and thesecondary heat treatment device 2. In this embodiment, the washingdevices 5 and 6 were provided because the primary heat treatment and thesecondary heat treatment both use oil cooling, but if water cooling, aircooling or gas cooling were used, then these washing devices would beunnecessary.

In a bearing component that has undergone heat treatment in the stepsdescribed above, since a nitrogen enriched layer (with a nitrogencontent of 0.1 to 0.7 wt %) is formed at the surface layer of thecomponent, a high hardness exceeding Hv700 can be achieved, and theaustenite grains within the micro structure are reduced in size to yieldan austenite grain size number exceeding 10. Furthermore, the breakingstress value for the bearing component is at least 2650 MPa, thehydrogen concentration within the steel is no more than 0.5 ppm, and theresidual austenite content within the steel is from 13 to 25%, whichrepresent far superior physical properties to conventional components.As a result of the above properties, the cracking resistance and theabrasion resistance can be improved, and a marked improvement in theservice life relative to rolling fatigue can also be achieved.

In the present invention, as described above, both the heater 21 of thesecondary heat treatment device 2 and the tempering device 7 that isused following the secondary heat treatment use an induction heatingdevice such as a high frequency heating device, and provided inductionheating is used, the heating efficiency is better and the heating timeconsiderably shorter than those cases where atmospheric gas heating isconducted in an atmospheric furnace or the like, and moreover becauseinduction heating uses electrical energy, control of the heating outputis also very easy. Accordingly, by suitable control of the heatingoutput, either by varying the power input to the inductor of the heater21 and/or the tempering device 7, or by varying the heating time, theheating times required for the two heating steps can be easily balanced.As a result, the need to halt heat treated products within theproduction line can be minimized, and the down times required for thevarious machinery can be reduced, enabling an improvement in the heattreatment efficiency across the entire system.

Furthermore, induction heating offers a number of advantages includingthe ability to evenly heat each component on a piece by piece basis, theability to heat with improved heating efficiency and shorter heatingtimes, the ability to perform localized heating and the freedom todetermine the thickness of the hardened layer, and the ability toimprove the fatigue strength through surface compressive residual stressby enabling rapid heating and rapid cooling, and consequently byconducting induction heating in both the heater 21 and the temperingdevice 7, further reductions in the cost of the bearing components, andfurther improvements in the quality and the service life relative torolling fatigue can be obtained.

As follows is a description of a second embodiment of the presentinvention, in which bearing components of a deep groove ball bearingshown in FIG. 2 are used as examples of steel components.

FIG. 4 is a diagram showing a schematic illustration of the constructionof a heat treatment system according to this second embodiment. Thisheat treatment system comprises a primary heat treatment device 1 and asecondary heat treatment device 2. A bearing component produced by amolding process comprising forging and/or turning or the like (not shownin the drawings), is fed sequentially through the primary heat treatmentdevice 1 and the secondary heat treatment device 2, and undergoes atwo-stage heat treatment comprising heating and cooling within each ofthe devices.

The primary heat treatment device 1 comprises a heater 11, a cooler 12and a washing device 13. The heater 11 is formed, for example, from aheating furnace that heats the bearing component in an atmospheric gascomprising a carburizing gas containing added ammonia. Inside thisheater 11, the bearing component is heated at a primary heatingtemperature T1 (from 800° C. to 950° C., for example, 850° C.) exceedingthe A1 transformation point for a predetermined time (of 40 minutes forexample) (primary heating), as shown in FIG. 5, and this heating causesactivated nitrogen to diffuse into the surface layer of the bearingcomponent, thus forming a nitrogen enriched layer (in this example, acarbonitrided layer). This primary heating has a primary object offorming a nitrogen enriched layer at the surface of the component, andat least nitridation must occur, although carburization is notessential. However, depending on the conditions, carburization may beessential as well as nitridation, particularly in cases wheredecarburization is a concern, or cases where the carbon content of thesteel is insufficient and a satisfactory level of hardness cannot beachieved. Following heat treatment, as shown in FIG. 5, the bearingcomponent is cooled to below the Ms point by the cooler 12 (using oilcooling, for example), and is then transported to the washing device 13for washing and removal of the cooling liquid (the oil, for example). Inthe cooler 12, instead of cooling the component to a temperature belowthe Ms point, the component may also be held at a constant temperatureof approximately 500° C. Furthermore, in FIG. 4, a continuous heatingfurnace is shown as the heater 11 for the primary heat treatment device1, but as shown by the dotted lines in the same figure, a batch-typeheating furnace 11′ may also be employed. Similarly, in FIG. 4, oilcooling is used for the cooler 12 within the primary heat treatmentdevice 1, but as shown by the dotted lines in the same figure, a cooler12′ that uses air cooling or gas cooling, such as cooling with N₂ gas,may also be employed. In such a case, since adhesion of cooling liquidto the bearing component does not occur within the cooler 12′, thesubsequent washing device 13 can be omitted, and the system can bestructured so that the bearing components pass directly from the cooler12′ to the heater 21 of the secondary heat treatment device 2. This notonly enables the structure of the primary heat treatment device 1 to besimplified, but also shortens the processing time.

Having undergone heat treatment in the primary heat treatment device 1,the bearing component is transported to the secondary heat treatmentdevice 2 via a transport device such as a conveyor. The secondary heattreatment device 2 comprises a heater 21, a cooler 22, a washing device23 and a tempering device 24. The heater 21 is a device for heating thebearing component by induction heating (such as high frequency heating),and is formed from a high frequency heating device. In the heater 21,each bearing component is treated on a piece by piece basis, and asshown in FIG. 5, is subjected to induction heating at a secondaryheating temperature T2 exceeding the A1 transformation point, for apredetermined time. FIG. 5 shows a case where the secondary heatingtemperature T2 is lower than the primary heating temperature T1;however, the upper limit of the secondary heating temperature T2 mayexceed T1. In the induction heating, the heating temperature and theheating time can be precisely controlled and the treatment can beconducted within a short time. Consequently, the austenite crystalgrains within the microstructure of the bearing components can beminiaturized. Furthermore, because heating of each bearing component isconducted on a piece by piece basis, non-uniformity in the heattreatment quality within each individual bearing component, andvariations in the quality of the heat treatment across different bearingcomponents can be minimized. As shown in FIG. 5, following heattreatment, the bearing component is cooled to below the Ms point (usingoil cooling, for example) in the cooler 22, and is then transported tothe washing device 23 for washing and removal of the cooling liquid.Subsequently, the bearing component is transported to the temperingdevice 24, and is tempered at a suitable temperature T3 (180° C. forexample). The tempering device 24 may also be disposed separately fromthe secondary heat treatment device 2. Furthermore, if the cooler 22uses air cooling, gas cooling or water cooling, then the washing device23 can be omitted.

In a bearing component that has undergone heat treatment in the stepsdescribed above, a nitrogen enriched layer (with a nitrogen content of0.1 to 0.7 wt %) is formed at the surface layer of the component,meaning a high hardness exceeding Hv700 can be achieved, and theaustenite grains within the micro structure are reduced in size to yielda crystal grain size number exceeding 10. Furthermore, the breakingstress value for the bearing component is at least 2650 MPa, thehydrogen concentration within the steel is no more than 0.5 ppm, and theresidual austenite content within the steel is from 13 to 25%, whichrepresent far superior physical properties to conventional components.As a result of the above properties, the cracking resistance and theabrasion resistance can be improved, and a marked improvement in theservice life relative to rolling fatigue can also be achieved.

As follows is a description of a third embodiment of the presentinvention, in which bearing components of a deep groove ball bearingshown in FIG. 2 are used as examples of the steel components.

FIG. 6 shows a schematic illustration of the construction of a heattreatment system according to this third embodiment. As shown in thefigure, this heat treatment system comprises a primary heat treatmentdevice 1, a secondary heat treatment device 2, washing devices 3 and 5,and a tempering device 6. A bearing component produced by a moldingprocess (not shown in the drawings) comprising forging, and then turningor the like, is fed sequentially through the primary heat treatmentdevice 1 and the secondary heat treatment device 2, and undergoesheating and cooling within each of the devices, as the primary heattreatment and the secondary heat treatment, respectively.

The primary heat treatment device 1 comprises a heater 11 and a cooler12. In FIG. 6, a continuous system is shown for the heater 11, but abatch-type furnace can also be used. The heater 11 is formed, forexample, from an atmospheric furnace that uses an atmospheric gascomprising a carburizing gas containing added ammonia. Inside thefurnace of this heater 11, a bearing component is heated at atemperature T1 (from 800° C. to 900° C., for example, 850° C.) exceedingthe A1 transformation point for a predetermined time of 40 minutes forexample (primary heating), as shown in FIG. 3. This heating causesactivated nitrogen to diffuse into the surface layer of the bearingcomponent, thus hardening that surface layer (gas carbonitriding). Theheater 11 has a primary object of forming a nitrogen enriched layer atthe surface of the component, and at least nitridation must occur,although carburization is not essential. However, depending on theconditions, carburization may be essential as well as nitridation,particularly in cases where decarburization is a concern, or cases wherethe carbon content of the steel is insufficient and a satisfactory levelof hardness cannot be achieved. The heater 11 may also use a vacuumfurnace, a salt bath furnace or an induction heater or the like.Following heat treatment, the bearing component is cooled to below theMs point by the cooler 12 (using oil cooling, for example), and is thentransported to the washing device 3 for washing and removal of thecooling liquid.

As shown in FIG. 6, a bearing component that has undergonecarbonitriding in the primary heat treatment device 1 is then suppliedto the secondary heat treatment device 2 via a transport device such asa conveyor. The secondary heat treatment device 2 is used for conductinghigh frequency quenching, and comprises a heater 21 and a cooler 22. Asshown in FIG. 3, the bearing component is then subjected to inductionheating (secondary heating) in the heater 21, at a temperature T2exceeding the A1 transformation point, for a predetermined time. Becausethis secondary heating is conducted within a short time by inductionheating, of course in the case where the heating is conducted at atemperature less than the primary heating temperature T1 and also evenin the case where the induction heating is conducted at a temperatureexceeding the primary heating temperature T1, the austenite grainswithin the steel can be reduced in size, by adjusting the heatingtemperature and the heating time. Following completion of this heating,the bearing component is transported to the cooler 22 and is cooled tobelow the Ms point (using oil cooling, for example) to effect quenching.Instead of conducting the cooling in a cooler 22 that is independentfrom the heater 21 as in the above example, the bearing component mayalso be cooled while still positioned at the induction heating locationinside the heater 21. In order to ensure satisfactory dimensionalprecision following the quenching, die quenching can be conducted in thesecondary heat treatment device 2 following the high frequency heating.Such die quenching enables an improvement in the precision ofthin-walled components such as the outer ring and the inner ring of aroller bearing, as well as components of varying thickness such as theouter ring and the inner ring of a tapered roller bearing, meaningfavorable bearing performance can be achieved with good stability. Diequenching refers to a treatment in which quenching is conducted with theheated article constrained by a die, and includes press quenching inwhich the article is constrained by applying pressure to the die.

Following completion of the secondary heat treatment, the bearingcomponent is removed from the secondary heat treatment device 2, washedin the washing device 5 to remove the cooling liquid, and is thentransported to the tempering device 6 where, as shown in FIG. 3,tempering is conducted at a suitable temperature T3 (180° C. forexample). In order to improve the treatment efficiency by shortening theheating time, this tempering is preferably conducted using inductionheating such as high frequency heating.

A sensor 9 that uses a non-contact method for detecting the temperature(the surface temperature) of the bearing component undergoing inductionheating is provided in the heater 21 of the secondary heat treatmentdevice 2. Examples of sensors that can be used as this sensor 9 includeinfrared temperature sensors. Inside the heater 21, the bearingcomponent is supported with a predetermined clearance relative to aninductor that is not shown in the figure, and the sensor 9 measures thetemperature of the supported bearing component using a non-contactmethod, and transmits the detected value to a control device 8. Usingthe detected temperature data, the control device 8 determines whetheror not the bearing component undergoing heating has reached thepredetermined secondary heating temperature T2, and whether or not thecomponent temperature is within a predetermined temperature range, andbased on the results of these determinations, conducts feedback controlof the induction heater 21. Control of the induction heater 21 is mainlyachieved by altering either the power supplied to the inductor or theheating time.

In the description above, oil cooling was used as the cooling methodwithin the primary heat treatment device 1 and the secondary heattreatment device 2, but other cooling methods such as water cooling, aircooling or gas cooling can also be employed, and different coolingmethods may also be used for the primary heat treatment device 1 and thesecondary heat treatment device 2. In this embodiment, the washingdevices 3 and 5 were provided because the primary heat treatment and thesecondary heat treatment both use oil cooling, but if water cooling, aircooling or gas cooling were used, then these washing devices would beunnecessary.

In a bearing component that has undergone heat treatment in the stepsdescribed above, a nitrogen enriched layer (with a nitrogen content of0.1 to 0.7 wt %) is formed at the surface layer of the component,meaning a high hardness exceeding Hv700 can be achieved, and theaustenite grains within the micro structure are reduced in size to yieldan austenite grain size number exceeding 10. Furthermore, the breakingstress value for the bearing component is at least 2650 MPa, thehydrogen concentration within the steel is no more than 0.5 ppm, and theresidual austenite content within the steel is from 13 to 25%, whichrepresent far superior physical properties to conventional components.As a result of the above properties, the cracking resistance and theabrasion resistance can be improved, and a marked improvement in theservice life relative to rolling fatigue can also be achieved.

The heater 21 of the secondary heat treatment device 2 is an inductionheater that uses an electromagnetic induction phenomenon to convertelectrical energy directly to thermal energy inside the steel structure,thus heating the steel. Consequently by adjusting the heating conditionssuch as the power input to the inductor or the heating time, the heatingoutput can be controlled simply and precisely. Accordingly, by operatingthe heating conditions of the heater 21 under feedback control from thecontrol device 8, based on the detected values from the sensor 9, thesecondary heating temperature T2 can be held reliably within apredetermined temperature range. Furthermore, because induction heatingis a piece by piece heating method, the types of heating irregularitiesthat can occur with the use of atmospheric furnaces, caused by thecharging location within the furnace, do not arise. Accordingly, a steelcomponent in which the crystal grain size has been reduced uniformlythroughout the entire component can be obtained, and the characteristiceffects of the above type of two-stage heat treatment, namely favorableabrasion resistance and cracking resistance, or an improvement in theservice life relative to rolling fatigue, can be achieved with goodstability.

Furthermore, induction heating offers additional advantages, such as theability to perform localized heating and the freedom to determine thethickness of the hardened layer, and the ability to improve the fatiguestrength through surface compressive residual stress by enabling rapidheating and rapid cooling, and consequently further reductions in thecost of the bearing components, and further improvements in the qualityand the service life relative to rolling fatigue can be obtained.

As follows is a description of a fourth embodiment of the presentinvention, in which a bearing component is used as an example of thesteel component.

FIG. 7 shows a schematic illustration of the construction of a heattreatment system according to this fourth embodiment. As shown in thefigure, this heat treatment system comprises a primary heat treatmentdevice 1, a secondary heat treatment device 2, two washing devices 3 and5, and a tempering device 6. A bearing component produced by a moldingprocess (not shown in the drawings) comprising forging, and then turningor the like, is fed sequentially through the primary heat treatmentdevice 1 and the secondary heat treatment device 2, and undergoesheating and cooling within each of the devices, as the primary heattreatment and the secondary heat treatment respectively.

The term “bearing component” refers to a bearing component of a rollingbearing such as a ball bearing, a tapered roller bearing, a rollerbearing or a needle roller bearing. As one example, FIG. 8 shows atapered roller bearing 4 comprising, as the main structural elements, anouter ring 41, an inner ring 42, and rolling elements (tapered rollers)43, and of these structural elements, in this description, the term“bearing components” describes those elements that are subjected torolling contact with an opposing member, namely the outer ring 41, theinner ring 42 and the rolling elements 43. Examples of the materialsthat can be used for these bearing components include bearing steel suchas SUJ2, as well as high temperature bearing steel comprising C: 0.6 to1.3 wt %, Si: 0.3 to 3.0 wt %, Mn: 0.2 to 1.5 wt %, Cr: 0.3 to 5.0 wt %,and Ni: 0.1 to 3 wt % (and preferably also comprising Mo: 0.05 to 0.25wt % and V: 0.05 to 1.0 wt %), and medium carbon steel comprising C: 0.4to 0.8 wt %, Si: 0.2 to 0.9 wt %, Mn: 0.7 to 1.3 wt % and Cr: no morethan 0.7 wt %.

The primary heat treatment device 1 comprises a heater 11 and a cooler12. In FIG. 7, a continuous system is shown for the heater 11, but abatch-type furnace can also be used. The heater 11 is formed, forexample, from an atmospheric furnace that uses an atmospheric gascomprising a carburizing gas containing added ammonia. Inside thisheater 11, a bearing component is heated at a temperature T1 (from 800°C. to 900° C., for example, 850° C.) exceeding the A1 transformationpoint for a predetermined time of 40 minutes for example (primaryheating), as shown in FIG. 3, and this heating causes activated nitrogento diffuse into the surface layer of the bearing component, thushardening that surface layer (gas carbonitriding). The heater 11 has aprimary object of forming a nitrogen enriched layer at the surface ofthe component, and at least nitridation must occur, althoughcarburization is not essential. However, depending on the conditions,carburization may be essential as well as nitridation, particularly incases where decarburization is a concern, or cases where the carboncontent of the steel is insufficient and a satisfactory level ofhardness cannot be achieved. The heater 11 may also use a vacuumfurnace, a salt bath furnace or an induction heater or the like.Following heat treatment, the bearing component is cooled to below theMs point by the cooler 12 (using oil cooling, for example), and is thentransported to the washing device 3 for washing and removal of thecooling liquid.

As shown in FIG. 7, a bearing component that has undergonecarbonitriding in the primary heat treatment device 1 (in the figure, anouter ring 41 of a tapered roller bearing is shown as an example) isthen supplied to the secondary heat treatment device 2, which conductsinduction heating such as high frequency heating. The bearing component41 is positioned within the inner periphery of an inductor 21, and asshown in FIG. 3, is then subjected to induction heating (secondaryheating) at a temperature T2 exceeding the A1 transformation point. Bycontrolling heating conditions such as a heating temperature and aheating time, the austenite grains within the steel are reduced in size.Following completion of this heating, as shown in FIG. 7, the bearingcomponent 41 is cooled to below the Ms point using a cooling liquid suchas oil in the state where the bearing component 41 is fitted into andheld by a die 22, thus effect quenching (die quenching). The quenchingmay be conducted by spraying the cooling liquid such as oil fromapertures provided in various locations within the die 22.Alternatively, instead of conducting the cooling at the inductionheating location as shown in the figure, the bearing component may alsobe transported to a separate location from the induction heatinglocation for cooling.

Following completion of the secondary heat treatment, the bearingcomponent is washed in the washing device 5 to remove the coolingliquid, and is then transported to the tempering device 6 and temperedat a suitable temperature T3 (180° C. for example). In order to improvethe treatment efficiency by shortening the heating time, this temperingis preferably conducted using induction heating such as high frequencyheating.

In the description above, oil cooling was used as the cooling methodwithin the primary heat treatment device 1 and the secondary heattreatment device 2, but other cooling methods such as water cooling, aircooling or gas cooling can also be employed, and different coolingmethods may also be used for the primary heat treatment device 1 and thesecondary heat treatment device 2. In this embodiment, the washingdevices 3 and 5 were provided because the primary heat treatment and thesecondary heat treatment both use oil cooling, but if water cooling, aircooling or gas cooling were used, then these washing devices would beunnecessary.

In a bearing component that has undergone heat treatment in the stepsdescribed above, a nitrogen enriched layer (with a nitrogen content of0.1 to 0.7 wt %) is formed at the surface layer of the component,meaning a high hardness exceeding Hv700 can be achieved, and theaustenite grains within the micro structure are reduced in size to yieldan austenite grain size number exceeding 10. Furthermore, the breakingstress value for the bearing component is at least 2650 MPa, thehydrogen concentration within the steel is no more than 0.5 ppm, and theresidual austenite content within the steel is from 13 to 25%, whichrepresent far superior physical properties to conventional components.As a result of the above properties, the cracking resistance and theabrasion resistance can be improved, and a marked improvement in theservice life relative to rolling fatigue can also be achieved.

In this embodiment of the present invention, induction heating and diequenching are conducted within the secondary heat treatment device 2, asdescribed above. In this case, theoretically, because the inductionheating causes minimal thermal distortion, and the quenching followingthe heating process is conducted as die quenching, a bearing componentwith little thermal distortion and a high level of dimensional precisioncan be produced at low cost, and a favorable level of dimensionalprecision can be achieved even for thin-walled components such as theouter ring or the inner ring of a ball bearing, or for components withvarying thickness such as the outer ring 41 or the inner ring 42 of atapered roller bearing. Accordingly, the quality of the bearingcomponents can be improved, meaning favorable bearing performance can beachieved with good stability.

Furthermore, induction heating offers additional advantages, such as theability to heat each individual structural component uniformly on apiece by piece basis, the ability to heat with improved heatingefficiency and shorter heating times, the ability to perform localizedheating and the freedom to determine the thickness of the hardenedlayer, and the ability to improve the fatigue strength through surfacecompressive residual stress by enabling rapid heating and rapid cooling,and consequently further reductions in the cost of the bearingcomponents, and further improvements in the quality and the service liferelative to rolling fatigue can be obtained.

As follows is a description of a fifth embodiment of the presentinvention, in which bearing components of a tapered roller bearing shownin FIG. 8 are used as examples of steel components.

FIG. 9 shows a schematic illustration of the construction of a heattreatment system according to this fifth embodiment. As shown in thefigure, this heat treatment system comprises a primary heat treatmentdevice 1, a washing device 3, and a plurality of secondary heattreatment devices 2, washing devices 5, and tempering devices 6, whichare arranged in parallel relative to the primary heat treatment device 1and the washing device 3. Bearing components produced by a moldingprocess (not shown in the drawings) comprising forging, and then turningor the like, are fed sequentially through the primary heat treatmentdevice 1 and the secondary heat treatment devices 2, and undergo heatingand cooling within each of the devices, as the primary heat treatmentand the secondary heat treatment respectively. A separate washing device5 and tempering device 6 are positioned as subsequent stages to each ofthe plurality of secondary heat treatment devices 2.

The primary heat treatment device 1 comprises a heater 11 and a cooler12. In FIG. 9, a continuous system is shown for the heater 11, but abatch-type furnace can also be used. The heater 11 is formed, forexample, from an atmospheric furnace that uses an atmospheric gascomprising a carburizing gas containing added ammonia. Inside thisheater 11, a bearing component is heated at a temperature T1 (from 800°C. to 900° C., for example, 850° C.) exceeding the A1 transformationpoint for a predetermined time of 40 minutes for example (primaryheating), as shown in FIG. 3. This heating causes activated nitrogen todiffuse into the surface layer of the bearing component, thus hardeningthat surface layer (gas carbonitriding). The heater 11 has a primaryobject of forming a nitrogen enriched layer at the surface of thecomponent, and at least nitridation must occur, although carburizationis not essential. However, depending on the conditions, carburizationmay be essential as well as nitridation, particularly in cases wheredecarburization is a concern, or cases where the carbon content of thesteel is insufficient and a satisfactory level of hardness cannot beachieved. The heater 11 may also use a vacuum furnace, a salt bathfurnace or an induction heater or the like. Following heat treatment,the bearing component is cooled to below the Ms point by the cooler 12(using oil cooling, for example), and is then transported to the washingdevice 3 for washing and removal of the cooling liquid.

As shown in FIG. 9, bearing components (in the figure, outer rings 41 ofa tapered roller bearing are shown as examples) that have undergonecarbonitriding in the primary heat treatment device 1 are divided up andsupplied, via a transport device such as a conveyor that is not shown inthe figure, to one of the secondary heat treatment devices 2 whichconduct induction heating such as high frequency heating. Within each ofthe secondary heat treatment devices 2, a bearing component 41 is heldwithin the inner periphery of an inductor 21, and as shown in FIG. 3, isthen subjected to induction heating (secondary heating) at apredetermined temperature T2 exceeding the A1 transformation point.Because this secondary heating is conducted within a short time, theaustenite grains within the steel can be reduced in size, irrespectiveof whether the heating temperature T2 is higher or lower than theprimary heating temperature, by adjusting the heating temperature andthe heating time. Following completion of this heating, the bearingcomponent 41 is fitted into the die 22, and cooled to below the Ms pointusing a cooling liquid such as oil, thus effect quenching (diequenching). The quenching may be conducted by spraying the coolingliquid such as oil from apertures provided in various locations withinthe die 22. Alternatively, instead of conducting the cooling with thecomponent held at the induction heating location as shown in the figure,the bearing component may also be transported to a separate locationfrom the induction heating location for cooling.

Following completion of the secondary heat treatment, the bearingcomponents are removed from each of the secondary heat treatment devices2, washed in the corresponding washing devices 5 to remove the coolingliquid, and are then transported to the corresponding tempering devices6 and tempered at a suitable temperature T3 (180° C. for example), asshown in FIG. 3. In order to improve the treatment efficiency byshortening the heating time, this tempering is preferably conductedusing induction heating such as high frequency heating.

In the description above, oil cooling was used as the cooling methodwithin the primary heat treatment device 1 and the secondary heattreatment devices 2, but other cooling methods such as water cooling,air cooling or gas cooling can also be employed, and different coolingmethods may also be used for the primary heat treatment device 1 and thesecondary heat treatment devices 2. In this embodiment, the washingdevices 3 and 5 were provided because the primary heat treatment and thesecondary heat treatment both use oil cooling, but if water cooling, aircooling or gas cooling were used, then these washing devices would beunnecessary.

In a bearing component that has undergone heat treatment in the stepsdescribed above, a nitrogen enriched layer (with a nitrogen content of0.1 to 0.7 wt %) is formed at the surface layer of the component,meaning a high hardness exceeding Hv700 can be achieved, and theaustenite grains within the micro structure are reduced in size to yieldan austenite grain size number exceeding 10. Furthermore, the breakingstress value for the bearing component is at least 2650 MPa, thehydrogen concentration within the steel is no more than 0.5 ppm, and theresidual austenite content within the steel is from 13 to 25%, whichrepresent far superior physical properties to conventional components.As a result of the above properties, the cracking resistance and theabrasion resistance can be improved, and a marked improvement in theservice life relative to rolling fatigue can also be achieved.

In this embodiment of the present invention, as described above, bearingcomponents 41 that have passed through a common primary heat treatmentdevice 1 are divided up and subjected to simultaneous secondary heatingwithin a plurality of secondary heat treatment devices 2, and highfrequency heating is performed within each of these secondary heattreatment devices 2, which enables the heating to be completed within ashort time period. As a result, the heating efficiency of the secondaryheat treatment can be improved dramatically, meaning the heat treatmentefficiency within the primary heat treatment and the secondary heattreatment can be balanced, enabling an improvement in the heatingefficiency across the entire system. The number of secondary heattreatment devices 2 can be selected in accordance with the difference inheat treatment efficiency between the primary heat treatment and thesecondary heat treatment, and can be any number that enables a balancebetween the two treatments. In the embodiment described above, only asingle primary heat treatment device 1 was provided, but a plurality ofthese devices can also be provided in parallel, and the heat treatmentefficiency then balanced with a plurality of secondary heat treatmentdevices 2 (in such cases, the number of secondary heat treatment devices2 is greater than the number of primary heat treatment devices 1).

Furthermore, because the high frequency heating conducted in thesecondary heat treatment devices 2 theoretically causes minimal thermaldistortion, and because the quenching following the heating process isconducted as die quenching, bearing components with little thermaldistortion and a high level of dimensional precision can be produced atlow cost, and a favorable level of dimensional precision can be achievedeven for thin-walled components such as the outer ring or the inner ringof a ball bearing, or for components with varying thickness such as theouter ring 41 or the inner ring 42 of a tapered roller bearing.Accordingly, the quality of the bearing components can be improved,meaning favorable bearing performance can be achieved with goodstability.

Furthermore, induction heating offers additional advantages, such as theability to heat each individual structural component uniformly on apiece by piece basis, the ability to perform localized heating and thefreedom to determine the thickness of the hardened layer, and theability to improve the fatigue strength through surface compressiveresidual stress by enabling rapid heating and rapid cooling, andconsequently further reductions in the cost of the bearing components,and further improvements in the quality and the service life relative torolling fatigue can be obtained.

In the above descriptions of each of the embodiments, a bearingcomponent was used as an example of the target article undergoing heattreatment, but the present invention is not restricted to bearingcomponents, and can be applied to a wide range of mechanical componentsthat require a favorable service life relative to rolling fatigue orsuperior cracking resistance, such as the structural components ofconstant velocity universal joints, or even general steel components.

Furthermore, the values of the primary heating temperature T1, thesecondary heating temperature T2 and the tempering temperature T3detailed above in each of the embodiments represent the values in thecase where bearing steel SUJ2 is used as the steel material. Thesetemperatures T1, T2 and T3 may differ from the values listed abovedepending on the nature of the steel material used.

1. A heat treatment system comprising: a primary heat treatment devicefor heating a steel component at a temperature exceeding the A1transformation point and then cooling the component to a temperatureless than the A1 transformation point, thus forming a nitrogen enrichedlayer at the surface of the component; and a secondary heat treatmentdevice for heating the steel component that has undergone primary heattreatment, at a temperature exceeding the A1 transformation point, andthen cooling the component to a temperature less than the A1transformation point, wherein the secondary heat treatment deviceincludes an induction heater, and tempering is performed by inductionheating following cooling within the secondary heat treatment device. 2.A heat treatment system comprising: a primary treatment device forheating a steel component at a primary heating temperature exceeding theA1 transformation point and then cooling the component to a temperatureless than the A transformation point, thus forming a nitrogen enrichedlayer at the surface layer of the component; and a secondary treatmentdevice for heating the steel component that has undergone heat treatmentby the primary treatment device, at a secondary heating temperatureexceeding the A1 transformation point, and then cooling the component toa temperature less than the A1 transformation point.
 3. A heat treatmentsystem as claimed in claim 2, wherein the primary treatment deviceincludes a heater for conducting gas carbonitriding.
 4. A heat treatmentsystem as claimed in claim 2, wherein die quenching is conducted withinthe secondary treatment device.
 5. A heat treatment system comprising: aprimary heat treatment device for heating a steel component at atemperature exceeding the A1 transformation point and then cooling thecomponent to a temperature less than the A1 transformation point, thusforming a nitrogen enriched layer at the surface of the component, and asecondary heat treatment device for heating the steel component that hasundergone primary heat treatment, at a temperature exceeding the A1transformation point, and then cooling the component to a temperatureless than the A1 transformation point, wherein induction heating isconducted in the secondary heat treatment device, the temperature of thesteel component undergoing induction heating is detected, and theinduction heater is operated under feedback control based on thedetected temperature value.
 6. A heat treatment system as claimed inclaim 5, wherein the temperature of the steel component undergoinginduction heating is detected using a non-contact type temperaturesensor.
 7. A heat treatment system comprising: a primary heat treatmentdevice for heating a steel component at a temperature exceeding the A1transformation point and then cooling the component to a temperatureless than the A1 transformation point, thus forming a nitrogen enrichedlayer at the surface of the component; and a secondary heat treatmentdevice for heating the steel component that has undergone primary heattreatment, at a temperature exceeding the A1 transformation point, andthen cooling the component to a temperature less than the A1transformation point, wherein the secondary heat treatment deviceconducts induction heating and die quenching.
 8. A heat treatment systemcomprising: a primary heat treatment device for heating a steelcomponent at a temperature exceeding the A1 transformation point andthen cooling the component to a temperature less than the A1transformation point, thus forming a nitrogen enriched layer at thesurface of the component, and a secondary heat treatment device forheating the steel component that has undergone primary heat treatment,at a temperature exceeding the A1 transformation point, and then coolingthe component to a temperature less than the A1 transformation point,wherein a plurality of secondary heat treatment devices are disposed inparallel.
 9. A heat treatment system as claimed in claim 8, whereininduction heating is conducted within each of the secondary heattreatment devices.
 10. A heat treatment system as claimed in claim 8,wherein die quenching is conducted within each of the secondarytreatment devices.
 11. A heat treatment system as claimed in claim 1,wherein gas carbonitriding is conducted within the primary heattreatment device.
 12. A heat treatment system as claimed in claim 5,wherein gas carbonitriding is conducted within the primary heattreatment device.
 13. A heat treatment system as claimed in claim 7,wherein gas carbonitriding is conducted within the primary heattreatment device.
 14. A heat treatment system as claimed in claim 8,wherein gas carbonitriding is conducted within the primary heattreatment device.
 15. A heat treatment system as claimed in claim 3,wherein die quenching is conducted within the secondary treatmentdevice.
 16. A heat treatment system as claimed in claim 9, wherein diequenching is conducted within each of the secondary treatment devices.